New Year rolls round a second too late

TOM WILKIE

Science Editor

Before the Scots set out in the dying moments of Hogmanay to "first foot" their neighbours as the bells ring in the New Year, they will have to pause for a second - exactly a second.

Although it may not seem likely to anyone who has already been at the festive drams, the earth is spinning round more slowly than it should with the result that the year is taking longer to go by.

Scientists have decided that a "leap second" should be added to the national timescale at midnight on New Year's Eve, delaying the start of 1996 by one second.

The "Greenwich" Time Signal to mark the transition between 1995 and 1996 will be exceptional, in that it will contain six instead of the usual five short pips before the start of the long pip which marks the hour.

The leap second is being inserted into national timescales at the same instant world-wide, so the Japanese will enjoy their extra second at 9am on their New Year's Day while New Yorkers will get theirs even before the old year has finished, at 7pm on New Year's Eve.

For this extra second of their festivities, the Scots can thank the French, for the decision to change our time has been made by the Paris-based International Earth Rotation Service. Researchers there time the rotation of the earth against hyper-accurate "atomic" clocks and, if the earth gets out of step with the regularity of the atomic world, they dictate that leap seconds should be inserted (or, sometimes, subtracted) from the time that the rest of us keep.

The sad truth is that Greenwich Mean Time is no more. Whether we know it or not, we have been setting our watches by Co-ordinated Universal Time (UTC) since 1972.

The introduction of UTC followed a decision five years earlier that a second of time should be defined in terms of the vibration of caesium atoms. Time according to this atomic clock can be measured to an accuracy better than one second in 300,000 years and it was not long before researchers detected discrepancies between "old" Universal Time, defined in terms of the rotation of the earth, and atomic seconds.

Tidal 'friction" from the effects of the gravitational pull of the sun and the moon, combined with internal inhomogeneities of the earth's composition, mean that it can speed up or slow down in its rotation. For as long as a second was defined in terms of the earth's rotation, these fluctuations, although detectable, were inherently measurable but they showed up against the better-than-metronomic atomic clocks. A reconciliation was needed and UTC was the result.

Since the switch to UTC, the earth has proved pretty erratic. The end- 1995 leap second will be the twentieth since Co-ordinated Universal Time began in 1972.